This invention pertains to the field of semiconductor material preparation and device fabrication. In preferred embodiments, the invention pertains to the field of electroluminescent materials and devices.
As pertinent to this invention, it is known in the prior art that nitrogen may be introduced into gallium phosphide (GaP) to create isoelectronic traps which function as radiative recombination centers for enhancement of the emission of green light when fabricated into junction devices. The prior art processes specifically designed for introducing nitrogen into the GaP, whether used as substrate or as an epitaxial film or both in the fabricated device, has been limited, apparently, to solution growth or liquid phase epitaxial processes. Typical of these prior art processes is that described, for example, in U.S. Pat. No. 3,462,320, where electroluminescent GaP devices are prepared by adding gallium nitride (GaN) and polycrystalline GaP containing a dopant of one conductivity type to a melt of elemental gallium (Ga) and heated to 1,200.degree. C in a sealed quartz ampoule, followed by cooling to 800.degree. C over a period of about 10 hours. The irregularly-shaped single crystals of nitrogen-doped GaP formed in the process is extracted from the gallium by washing in concentrated HCl, cut to size and shape and polished. The product thus formed is used as a substrate onto which an epitaxial layer of GaP of different conductivity type is grown by the liquid phase technique known as tipping. Contacts are affixed to the P and N regions to fabricate a two-terminal P-N junction device.
In other prior art processes a nitrogen-doped GaP epitaxial film is grown by liquid phase epitaxial deposition, e.g., by tipping, onto a substrate of GaP of opposite conductivity type to that in the epitaxial film; the GaP substrate may or may not be further doped with nitrogen.
It is also known to prepare electroluminescent GaP diodes by vapor phase processes. However, there seems to be no disclosure in the prior art specifically teaching the intentional doping of GaP with nitrogen in vapor phase processes to produce electroluminescent materials suitable for light-emitting diodes. In one known process, sulfur-doped GaP was epitaxially deposited from the vapor phase onto a gallium arsenide (GaAs) substrate by a phosphorus trichloride (PCl.sub.3) transport process. In that process, purified hydrogen carrying the PCl.sub.3 was combined with a stream of hydrogen carrying the sulfur impurity and the gaseous mixture introduced into a quartz reactor tube to react with Ga at 930.degree. C and form GaP which was epitaxially deposited onto the GaAs substrate. Thereafter, a P-type dopant, e.g., zinc or beryllium, was diffused into the N-type GaP layer to form a P-N junction. The emission spectra for diodes fabricated from the epitaxial GaP/GaAs structure showed, inter alia, that isolated atoms of nitrogen were present as an unintentionally added impurity; no comment is offered as to either the possible source of nitrogen addition or its location within the device material, i.e., whether in the P or N regions of the GaP. The process referred to is described in more detail by E. G. Dierschke et al. in the Journal of Applied Physics, Vol. 41, No. 1, pages 321-328, Jan., 1970.
The prior art relative to the incorporation of isoelectronic impurities into semiconductor materials does not appear to contain any positive disclosure relevant to the fabrication of electroluminescent devices from alloys (mixed crystals or solid solutions) of binary III-V compounds, such as gallium arsenide phosphide, GaAs.sub.1-x Px, where x has a value greater than .about.0.2 and less than one, produced in any manner.
In the Dierschke et al. article referred to above, reference is made to contamination of the epitaxial GaP layer with arsenic atoms, derived from the GaAs substrate, resulting in a composition of GaAs.sub.x P.sub.1- x, in which the mole fraction of arsenic, in the most representative curve for crystals grown under normal conditions, had a maximum value of 0.06 at the GaP/GaAs interface, decreasing with distance therefrom to a value of less than 0.02 at a distance of 1.0 mm. As noted above, the arsenic was introduced, unintentionally, into the epitaxial GaP from the GaAs substrate; the presence of arsenic in the GaP layer was unknown to the authors prior to an analysis of the emission spectra and verification by electron probe measurements. As further noted above, the Dierschke et al. article does not indicate whether the isolated atoms of nitrogen shown to be present by emission spectra, were present in the N-type or P-type GaP; in any event, the nitrogen, like the arsenic, was unintentionally added.
In a process described by P. J. Dean et al. in Applied Physics Letters, Vol. 14, No. 7, pages 210-212, Apr. 1, 1969, phosphorus-rich GaAs.sub.x P.sub.1-x (where x .ltorsim. 0.2) doped with nitrogen was grown from the vapor by introducing phosphine (PH.sub.3) and arsine (AsH.sub.3) in a stream of wet hydrogen into an open tube reactor heated to about 1,040.degree. C wherein the water reacted with sintered boron nitride (BN) to generate NH.sub.3 above the crystal growth zone; nitrogen from the NH.sub.3 was used to dope the GaAs.sub.x P.sub.1-x, apparently uniformly throughout the growing crystal. However, the article published by Dean et al., supra, was directed primarily to a discussion of the localization energy of excitons et isoelectronic nitrogen sites in phosphorus-rich GaAs.sub.x P.sub.1-x, based on experimental results from optical absorption spectra for x .ltorsim. 0.2. No disclosure is made in the Dean et al. article pertaining to the fabrication of electroluminescent gallium arsenide phosphide P-N junction devices or performance characteristics thereof.
In the prior art processes referred to above the isoelectronic impurity, nitrogen, is usually distributed uniformly throughout the epitaxial film and/or substrate upon which the film is deposited. Since the electroluminescence from isoelectronic nitrogen sites occurs within the vincinity of the P-N junction space charge region, nitrogen atoms in the remaining portions of the material absorb part of the emitted radiation. In order to obtain the desired nitrogen profile, it has been suggested that a liquid phase epitaxial double tipping technique be employed. In such proposed method, during the first tipping operation to grow a layer of one conductivity type, the epitaxial growth cooling cycle is interrupted after growth of a layer having a given nitrogen concentration, and the nitrogen content increased by adjusting the NH.sub.3 concentration to increase the GaN concentration in the Ga growth solution. On resuming the cooling cycle the subsequent layer growth would have the desired higher nitrogen concentration. Next, a layer of opposite conductivity type is grown by a second tipping operation from a melt conaining the desired GaN level. After a desired growth period, the cooling cycle is interrupted and GaN evaporated from the Ga growth melt. Upon resuming the cooling cycle, the remaining layer is grown with a low nitrogen level.
Therefore, it is an object of this invention to provide a vapor phase process for the preparation of nitrogen-doped GaAs.sub.1-x P.sub.x electroluminescent materials.
It is a further object of this invention to provide a simple means for introducing nitrogen into a specified region of the epitaxial layer of GaAs.sub.1-x P.sub.x.
A further object of the invention is to provide a new composition of matter particularly suitable for use in the fabrication of electroluminescent devices.
Another object of this invention is to provide improved electroluminescent devices fabricated from the nitrogen-doped GaAs.sub.1-x P.sub.x produced herein.
These and other objects will become apparent from the detailed description of the invention given below.